Any Interest in 6mm ? - Build Thread

[rsterne]

More than that I think.... I still have to chamber, port, crown and sleeve the barrels (still have a 6 mm reamer to make) and install them in the receivers.... Plus there is a lot of machining left on the receivers.... bolt "J" slots, port adjuster, loading area, barrel retention.... and make the bolts, handles and loading platform inserts.... The receivers will be the same, but there will be different loading platform inserts for the 6 mm and .257 cal.... In addition, after the first test I will have to make the second set of guts for the other valve.... and hopefully not have to redesign it....

Plus I'm waiting for a regulator.... *poke*....

Bob

[rsterne]

Today I milled out the loading port area in the two receivers.... Instead of milling it straight across, I left the left-hand side of the receiver intact from the boreline over, so that I don't lose so much rigidity.... These guns are single-shot, so no need to cut it away more than that.... I did this on my Monocoque and it works well, but of course you must load the bullet from the right side.... Drilling a 1/2" hole straight through the receiver to assure the alignment of the bolt and barrel means that when you machine the loading port you don't have a tray to guide the bullet into the bore.... so that is made from a separate piece of 1/2" diameter steel.... I drilled one for 6 mm and the other for .257 before milling them to shape.... You can see one of the inserts sitting on top of the receiver in the foreground.... along with a flat-head 4-40 screw which holds it in place.... The back receiver already has one installed....



With the loading tray in place, the right side is level with the centerline of the bore.... and the left side is vertical, tangent to the left side of the bore.... You don't need to look at what you are doing to load, just place the bullet on the right hand lip of the tray, and roll it to the left and close the bolt....

Bob

[rsterne]

I got lots more done on the receivers today.... This reduced the weight of them quite a bit.... and vastly improves the appearance....



The top and upper edges are now milled down, leaving raised areas where the dovetails for the scope mounts will be.... I also bevelled the top corners as well.... When I machine the dovetails I will be angling them down 20-25 MOA at the front to put the barrel on POA at 100 yards with the scope still optically centered....

Bob

[YEMX]

Excellent Bob!  As always!

[Prouzy]

Coming along nicely, looking forward to more!

[MJP]

Looks good Bob, nice to make parts in series.

Marko

[rsterne]

I worked on the receivers more today.... I drilled and tapped the six holes for the set-screws that will secure the barrel.... The original plan was for 10-32 screws at 120 deg. to each other.... but I was unable to get them of the proper length here in town, but I was able to get 5 mm x 0.8 mm Metric setscrews that are 10 mm long.... You can enlarge 10-32 threads to M5 by simply running the tap through them, as they are the same pitch (within less than 1%) and the Metric screws are 0.008" larger than the 10-32s.... I dislike mixing SAE and Metric fasteners on a project, but the alternative would have been to wait weeks to get the 10-32s.... I justified it by the fact that the barrels are 14 mm and the CF sleeve is 20 mm OD.... so at least everything to do with the barrels is Metric.... 



I had one other little glitch when setting up to drill the lower holes for the setscrews.... In order for the receiver to clear the carriage on my lathe (I was using my milling attachment to drill and tap the holes accurately) I could not drill them 30 deg. below the horizontal, I had to use 35 deg.... This is of no practical importance, but means that the angle from the top to each side screw is 125 deg. and the angle between the bottom screws is 110 deg.... instead of them all being 120 deg....

I also set up and milled the slots for the bolt handle today.... The front receiver is finished, but dinner hit the table before I got the vertical part of the rear slot done.... I'll do that tomorrow before I set up to fly-cut the recess in the front bottom of the receivers to locate the plastic pipe of the forestock....

Bob

[MJP]

Every build has a glitch, it's nothing new. I too hate mixing metric and sae fastners, thats why I dont use sae. 

Marko

[rsterne]

"Officially" Canada is on the Metric system.... but if you go into any Industrial Supply, you will find 90% SAE fasteners, and VERY few Metric.... That is why I always work in SAE, plus in the USA (where I also buy fasteners) Metric is even more rare.... I got caught out this time, went in to get the 10-32 setscrews and all they had was 10-24 (and I had already started tapping the holes 10-32).... I just got lucky that they had the M5 setscrews in the length I needed and I had the tap to convert them....

Bob

[rsterne]

Here are the receivers with everything now done except to machine the 20-25 MOA slope in the scope mounts and cut the dovetails in them....



On the lower front corner you can see the semi-circular recess milled to act as a clamp for the plastic pipe forestock.... It pinches the pipe between the bottom of the receiver and the main air tube, mounting it solidly at the back in the lowest possible position so that it doesn't rattle around, and clears the barrel.... The velocity adjusting screws are installed in the back, in line with the lower bolt slot.... The 8-32 SHCS pushes on a 1.5" long steel pin that can prevent the bolt handle from retracting fully in the slot.... I have about 10 turns of adjustment, the back receiver has the adjuster fully retracted (maximum velocity) and the one in front has the adjuster set to minimum velocity.... I labelled the steel pin which is sticking out into the lower bolt slot, which holds the bolt forward so that the nose obstructs the barrel port, reducing airflow (to almost zero when fully closed as shown).... You can't see it in these photos, but there is a vertical hole coming up from the bottom, in line with the bottom of the adjusting screw.... It is tapped for a 6-32 setscrew which presses on a 1/4" long piece of thin Teflon rod which acts as a brake to prevent the velocity adjusting screw from moving by itself....

Bob

[rsterne]

I got to work on the barrels today.... The first order of business was to machine the breech end to 1/2" OD for 1" of length so that it will fit into the receiver.... In addition, I had to machine down the OD of the carbon fibre tube slightly to fit inside the 25/32" forward section which is drilled to a depth of 2.5".... I did that with the end of the tube running on a live center in the tailstock of the lathe to insure concentricity between the ID and OD, and it worked out great.... Both the CF sleeve and the barrel slide perfectly into the receiver and can be rotated individually, showing they are concentric with each other and the holes in the receiver are as well.... This was a bit of a nervous time, as any inaccuracy there would have been hard to correct.... At the muzzle end I turned the barrel down to 1/2" as well and threaded it 1/2"-20 NF so that it can accept either a Hatsan Air Stripper (as a backup) or my intended Harmonic Tuner, which I have yet to make.... Here is a photo of the barrel ends, breech on top, muzzle below....



and here is a photo of them with the CF sleeve in place (but not bonded yet)....



You can see where I had to skin down the outside of the CF tube to fit the 25/32" hole in the receiver, just a few thou was all that was required.... The next job will be to cut the chambers in the barrels before bonding on the CF sleeves.... I think the reamer I made for my .257 Monocoque will work, but I will have to make a new chamber reamer for the 6 mm.... I received some samples from Nick at Nielsen Specialty Ammo in the mail today (perfect timing, thanks Nick), as I would like to make sure the chamber in the .257 will accept as many different bullets as possible.... Here is a photo of the selection of .257 bullets I now have to try....



Left to Right they are.... My 98 gr. BBT HP, the 91 gr. Noble from Nick and his 85 gr. rebated boattail HP, an 88 gr. RCBS 82301, an 82 gr. NOE 260-80 FN, and a 73 gr. Lyman 257420.... I can also cast HP versions of the 82301 and 257420, so I will have lots of bullets to choose from.... I don't expect the 98 gr. BBT (or the FN version of it) to work in the 10" twist of the TJs barrel I have.... so if the chamber has to be too short to fit all the other bullets for it to chamber, so be it.... but just looking at the position of the ogives in the above photo, I think I can cut a chamber that will work with all of them.... If anything, the bullet with the Ogive starting the furthest forward is Nick's swaged HPBT.... Here are the 6mm Bullets.... the TJs barrel I have is also 10" twist....



Left to Right they are.... 58 gr. Bowman HP from Arsenal, 59 gr. NOE 245-64 HP and 69 gr. NOE 245-74 HP.... I can also cast FN versions of all three bullets.... The weights of the FN versions are 61 gr., 63 gr. and 73 gr. respectively.... The 63 gr. NOE is a shortened, FB version of the 73 gr. beside it, with the GC shank removed.... I also have a mould for a 65 gr. shortened version of the NOE 260-80, done the same way, but that will be too light for the power of this .257 cal (I hope)....

Bob

[rsterne]

I worked on making the Harmonic Tuners today.... I got two of the brass weights made, and one of the mounts that screws onto the muzzle.... Here is what it looks like when assembled....



The threads are 15/16"-28.... I like 28 TPI for this job because 10 deg. of rotation is 0.001" of movement (1 "hour" on a clock face is 0.003", 1 turn is 0.036").... The brass weight is made from a piece of 1.5" bar stock 1" long, the setscrew presses on a short piece of Teflon rod which acts as a brake to prevent the weight from moving by itself.... The mount is made from a piece of 1" CRS.... There is a small collar of that diameter at the front, with a 3/16" hole drilled though it to allow it to be tightened onto the muzzle with a short piece of 3/16" bar stock.... The remainder is threaded, the 2" of threads giving over 1" of adjustment for the weight.... The center portion is threaded inside 1/2"-20 NF to thread onto the barrel, and the back is counterbored 0.80" ID for 1" of length to fit over the CF sleeve.... The front is drilled out 3/4" ID, it only projects 1/2" past the muzzle to allow the HPA from the shot to escape quickly without affecting the bullet.... and it also serves to protect the crown which is a simple 90 deg. angle....

The mount weighs a couple of ounces, and the movable brass weight weighs just over 5 oz. at the present time, although I may put chamfers on the corners for appearance after initial testing.... I'm really looking forward to trying this system, I know it is tremendously successful in rimfire benchrest, where it can drastically affect the group size....

Bob

[rsterne]

Here are the completed harmonic barrel tuners.... The muzzle of the barrel is about 3/8" inside the front of the mount, about where the outside threads start....



The scale is like a clock face, with the quarters having longer index marks.... with 12 o'clock being where the setscrew is.... Each mark equals 0.003" of movement on the tuning weight.... By starting with the weight against the shoulder at the front, you can record the position using "hours" for the turns and "minutes" for the index marks, so 9-1/3 turns I would record as 9:20.... and 15-3/4 turns as 15:45 etc.etc.... I'm really pleased with the way they turned out....

Bob

[PelletsForPests]

This is awesome Bob! Posting to keep updated on your progress

[Kinetic45^]

I am in awe on this project...
and want to thank you for your support of the community and taking the time and effort to expand the knowledge base of the HPA/PCP community.

[K.O.]

Still dreaming about finding a 26" Remmy 722 1:12 .244 barrel..

I think it would work well up to about  70g...

of course I keep watching..

[rsterne]

I cut the chamber in the .257 barrel today, using the reamer I made for my Monocoque....



Here are all the bullets that will fit.... They were all chambered to a depth of 0.60" from the end of the barrel, which will put the base just ahead of the front of the barrel port.... All bullets were sized to 0.257" before testing how they chambered.... When the chamber is too short, you sure know it, as you have to push hard enough to mark up the base, and it's pretty obvious when you get the length right....



I enhanced the contrast as much as possible so that you can see the rifling marks, but they are still hard to see on all except the longest two BBTs (112 gr. FN and 98 gr. HP), where they are clear on the back of the head.... The shortest bullet is the Lyman 257420 73 gr. and it barely touches the leade.... If you push it in by hand, you can only feel resistance for about the last 1/16", and basically cannot see any marks, even with a loupe, unless you push it in another 1/16", where you feel significant resistance.... I don't expect the two longest bullets to work in the 10" twist in this barrel.... but since I can chamber them I will be able to test that and prove or disprove the validity of the Kolbe Twist Calculator numbers for them.... Many of my BBTs seem to be stable in a slower twist than it gives, so there is a chance that the 98 gr. HP may work.... providing I have enough power to push it....

The other four bullets all slid in between 0.50-0.54" and then took a bit of a push (not much, mind you) to seat them to 0.60".... If you look carefully you can see shallow marks from the tapered leade, which they are sitting firmly against when chambered.... Those 4 bullets, from L to R, are the 91 gr. Noble, the 85 gr. NSA BTHP, the 88 gr. RCBS 82301, and the 82 gr. NOE 260-80 FN.... If I find one particular bullet shoots the best and want to play with different seating depths, I can do that by changing the length of the bolt nose....

Next step is to make the bolt, cut the O-ring groove and polish the chamber behind the barrel port.... then cut the CF sleeve to final length and glue it on.... and then machine the barrel port.... I was very pleased with the way the 3-flute reamer worked, so I am going to copy that design when I make the one for the 6mm barrel.... just smaller diameters for the pilot and chamber of course....

Bob

[Finn]

Odd numbered reamers usually work well, odder the better. I once made a 5 flute reamer and by accident got the flutes unevenly divided, so that the width of tooth wasn't even close to same but it worked so much better than the correctly made one that I ended up using that.  Btw what do you use for chamber polishing and what level of accuracy and surface finish ("RA"/μm) is achieved?

[PeterL]

Awesome work Bob. I'm just in awe of you work and attention to detail.

Peter

[rsterne]

I made the 6mm reamer this afternoon.... still not hardened and tempered but it turned out dimensionally accurate....



It is made from O1 Drill Rod.... The next step is to heat it to cherry red and quench it in oil to harden it.... Then I will polish it, hone the cutting edge again, and then temper it in the oven.... tomorrow's jobs....

Albert, I couldn't tell you what the finish on the chamber is.... Yes, I understand that unevenly space flutes actually work better than symmetrical ones.... but I just mill the flutes in mine in 5C collet in a hex block, skipping every other flat.... I plunge the end mill 5% of the caliber below the centerline to create the relief angle.... That creates a very fine cut, so cutting the chamber is slow.... I run the lathe at 28 RPM and continually pull the reamer to clean it of the dust sized metal particles the reamer creates....

Bob